An unknown advanced race built a gene splicer on the planet which would later be named Basingstoke, and it was believed that they were attempting to use the machine's gene-splicing effect to terraform Basingstoke. However, the terraforming operation apparently went awry as the gene-splicing process was too powerful to control, and the terraformers apparently became the progenitors of all life which would later develop on Basingstoke, due to the gene splicer's effect.
Some time after the failed terraforming operation on Basingstoke, Buchan Foster found and reactivated the gene splicer, causing the planet's native life descended from the terraformers, and the human convicts of the Van Diemens III, to begin to mix and take characteristics from each other again.
After the Eleventh Doctor and Conrad Finch found the gene splicer, the Doctor used his regenerative ability (which had been added into the gene-splicing ecosystem after the Doctor himself was affected by the genetic splicing effect) to trigger a biological reset into the spliced lifeforms which reverted the affected Van Diemens convicts to human. The Doctor subsequently reconfigured the gene splicer to redirect its energy back in on itself, permanently deactivating the machine and burying it in an explosion. (COMIC: Supernature)